Previous studies examining the development of the obese-hyperglycemic syndrome (C57BL/6J ob ob mice) have shown that most metabolic, behavioral, and endocrine abnormalities are manifested by the 4th to 5th week postnatally. In studies just completed of the preweaning development of the syndrome we have found that obesity, hyperinsulinemia, hypoglycemia, reduced skeletal growth and lower concentrations of thyroxine are evident in these animals as early as 6-10 days after birth. However, glucose intolerance (as measured by intraperitoneal glucose tolerance tests), insulin resistance (glucose response to IP insulin injection), and hyperglycemia do not appear until relatively late in the post-weaning period (23 days of age). Examination of the effects of daily oxytetracycline injections on the obese-hyperglycemic syndrome showed that the drug was ineffective in significantly altering the syndrome in weanling mice but as previously reported, substantially reduced the hyperglycemia, hyperinsulinemia, and elevated body weight gains of older ob/ob animals. We have also found that oxytetracycline treatment of fully mature obese mice normalizes carcass fat content while increasing lean body mass and reverses many of the hepatic abnormalities (increased triglyceride, cholesterol, total lipids; decreased glycogen) frequently reported in these mutant mice.